Several authors have already mentioned such methods.
The article entitled "Preparation of Bi.sub.1.5 Sr Ca Cu.sub.2 O.sub.x glass fibers using melt-quenched glasses" by M. Onishi et al., published in the Japanese Journal of Applied Physics, Vol. 28, No. 12, December 1989, pp. L 2204-L 2206, describes obtaining a flat fiber of length 120 cm, of width lying in the range 200 .mu.m to 1000 .mu.m, and of thickness lying in the range 25 .mu.m to 60 .mu.m. To do this, a flat preform is prepared of length 20 mm, of width 10 mm, and of thickness 20 mm, by melting the raw materials in a crucible, by quenching, and by annealing. The preform is suspended in an oven with a weight at one of its ends and fiber-drawing is performed. Superconductive properties are not obtained, but the article indicates that annealing should be provided at above 797.degree. C.
The article entitled "Thermal stability and fiber drawing in Bi.sub.x Pb.sub.y Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.z glasses" by Takayuki Komatsu et al., published in the Journal of Non-crystalline Solids, 126 (1990), pp. 273-276, investigates various compositions in which 1.6.ltoreq.x.ltoreq.4.0 and 0.2.ltoreq.y.ltoreq.16 and it is mentioned that it is possible to draw a fiber from a glass having the following composition Bi.sub.2.4 Pb.sub.0.5 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.z or Bi.sub.2.7 Pb.sub.0.4 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.z.
The raw materials are melted in a platinum crucible in the range 1150.degree. C. to 1250.degree. C.; the material is cast and quenched by being pressed on a metal plate; it is annealed at around 200.degree. C. to 300.degree. C., and a plate-shaped preform is obtained. With a weight at its bottom end, the preform is suspended in an oven heated in the range 550.degree. C. to 600.degree. C., and fiber-drawing is performed The resulting fibers are 50 cm long of width lying in the range 100 .mu.m to 200 .mu.m, and they are 20 .mu.m thick. Such fibers are not superconductive and they must be annealed at 840.degree. C. for 100 hours in order to obtain superconductivity with T.sub.c =84K. The authors indicate that optimum annealing conditions could give rise to a temperature T.sub.c greater than 100K, but it is observed that if annealing temperature or time is increased, then there is a danger of vaporizing the lead and of destabilizing the superconductive phase.
The article entitled "Bi-Pb-Sr-Ca-Cu-O superconducting fibers drawn from melt-quenched glass preforms" by M. Onishi et al., published in the Japanese Journal of Applied Physics, Vol. 29, No. 1, January 1990, pp. L 64-L 66, proposes a method analogous to the above method for a composition of the type Bi.sub.1.6 Pb.sub.0.4 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.z, referred to as "2223 phase". It underlines the fact that the critical temperature increases with increasing annealing temperature of the resulting fiber (e.g. 240 hours in air at 860.degree. C.). The optimum result given relates to a critical temperature T.sub.c =68K for a material which suffers from the following drawbacks:
a loss of lead which is used as a stabilizer in 2223 phase whose critical temperature T.sub.c is 110K;
a structure that is porous; and
no texturing.
An object of the present invention is to provide a fiber of a material having the composition Bi.sub.x Pb.sub.y Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.z by fiber-drawing and having a critical temperature that reaches an optimum value, which is at least greater than 90K.